1. Field of the Invention
The present invention relates to a nonreciprocal circuit device and a nonreciprocal circuit apparatus, which may be used as an isolator in the microwave band and the millimeter-wave band, and to a transceiver using the same.
2. Description of the Related Art
Hitherto, conventional isolators using an edge-guided mode have been disclosed in Japanese Unexamined Patent Publication No. 4-287403 and Japanese Unexamined Patent Publication No. 63-124602, incorporated by reference.
The former isolator includes a microstrip line formed on a magnetic base member and a strip conductor grounded at the middle to one side, in which an external DC magnetic field is applied to the magnetic base member in the perpendicular direction. The latter isolator includes a coplanar waveguide (hereinafter referred to as a xe2x80x9ccoplanar linexe2x80x9d) formed on a magnetic base member and an electromagnetic wave absorber formed from a central conductor of the coplanar line to one ground conductor, in which an external DC magnetic field is applied to the magnetic base member in the perpendicular direction. Both the former and the latter isolators generate an isolation effect by varying magnetic characteristics in the magnetic base member by means of the external DC magnetic field, causing the electromagnetic field distribution of a propagation mode at both sides of the line to be asymmetrical due to an edge-guiding effect, and selectively attenuating a propagating signal in accordance with the direction of the external magnetic field and the propagating direction of the signal.
The former isolator employs the microstrip line as a transmission line. When the isolator is provided on a planar circuit formed by the microstrip line, connectability of the circuit is relatively easy. The latter isolator employs the coplanar line as a transmission line, so that a transition between the coplanar line and, for example, a coaxial line is relatively simple.
However, the microstrip line and the coplanar line have relatively large transmission losses. When the transmission distance is long, and particularly, when low transmission loss is required, the microstrip line and the coplanar line are not suitable.
An alternative transmission line includes a cavity waveguide or a nonradiative dielectric waveguide that has low transmission loss. However, when these waveguides are used for forming a nonreciprocal circuit device such as an isolator, the overall size of the isolator must be large. On the other hand, when the nonreciprocal circuit device formed by the microstrip line or the coplanar line is used, a line transition element is required for transition between the microstrip line or the coplanar line and the cavity waveguide or the nonradiative dielectric waveguide. As a result, the overall size is not reduced, and conversion loss occurs.
The present invention is able to provide a nonreciprocal circuit device for eliminating or minimizing the above problems.
The present assignee has previously filed a patent application disclosing a planar dielectric transmission line in Japanese Unexamined Patent Publication 8-265007, published Oct. 11, 1996, corresponding to U.S. patent application Ser. No. 08/832,305 filed Apr. 3, 1997, now U.S. Pat. No. 5,986,527 issued Nov. 16, 1999, incorporated by reference. This planar dielectric transmission line includes opposing slots formed on both sides of a dielectric base member, and employs a region where the slots oppose each other with the dielectric base member therebetween as a propagation region. The planar dielectric line has very small transmission loss. The present invention employs this type of planar dielectric line to generate nonreciprocal circuit characteristics by means of the planar dielectric line alone.
According to one aspect of the present invention, there is provided a nonreciprocal circuit device including conductive films formed on both sides of a substrate which has ferrimagnetic characteristics, first and second slots formed respectively in the conductive films and opposing each other, and at least one resistive film formed on a corresponding one of the faces of the substrate near the corresponding slot. A DC magnetic field is applied to the substrate so as to be substantially parallel to the substrate and to be substantially perpendicular to the first and second slots, and the nonreciprocal circuit device is thereby obtained.
Alternatively, the substrate may be a dielectric member, and a magnetic member may be stacked in the dielectric member adjacent to the resistive film. Also, a second resistive film may be formed on the one face of the substrate on an opposite side of the corresponding slot from the first-mentioned resistive film.
According to another aspect of the present invention, there is provided a nonreciprocal circuit device including conductive films formed on both sides of a substrate which has ferrimagnetic characteristics, first and second slots formed respectively in the conductive films and opposing each other, and a resistive film formed on one side of the substrate near at least one of the first and second slots. A DC magnetic field is applied to the substrate so as to be substantially perpendicular to the substrate, and the nonreciprocal circuit device is thereby obtained.
Alternatively, the substrate may be a dielectric member, and a magnetic member may be stacked in the dielectric member adjacent to the resistive film. Also, a second resistive film may be formed on the other side of the substrate near the other of the first and second slots.
The above substrate having ferrimagnetic characteristics also serves as a dielectric member having a predetermined dielectric constant. The first and second slots operate as a planar dielectric transmission line in which the interior of the substrate sandwiched between the first and second slots serves as a propagation region. Specifically, the dielectric constant and the thickness of the substrate are determined so that electromagnetic waves propagate while being totally reflected from a first side of the substrate in the first slot and a second side of the substrate in the second slot. Accordingly, the first and second slots operate as a planar dielectric transmission line having very small transmission loss.
Preferably, the substrate is formed by stacking a magnetic member having ferrimagnetic characteristics and a dielectric member, and the conductive films are formed on the dielectric member. With this arrangement, connectability of the nonreciprocal circuit device with another planar circuit formed on the dielectric member is extremely easy. For example, when the nonreciprocal circuit device according to the present invention is provided on the dielectric member on which a planar circuit is formed, there is no need to employ a structure in which the planar circuit formed on the dielectric member and the nonreciprocal circuit device formed on the magnetic member are connected.
According to another aspect of the present invention, there is provided a nonreciprocal circuit device including conductive films formed on both sides of a dielectric member defining first and second slots which oppose each other, a magnetic member having ferrimagnetic characteristics being stacked on the dielectric member, and a resistive film, which opposes one of the areas of the first and second slots, formed on the magnetic member. A DC magnetic field is applied to the dielectric member and the magnetic member so as to be substantially parallel to the dielectric member and the magnetic member and to be substantially perpendicular to the first and second slots, and the nonreciprocal circuit device is thereby obtained.
As described above, even when the resistive film is separated from the conductive films, the electromagnetic field distribution of a propagation mode is localized (concentrated) toward the resistive film when a signal propagates in the blocking direction. Electrical power is consumed by the resistive film, and the signal is thereby attenuated. In this case, the resistive film is not required to form a slot, thus simplifying the patterning of the resistive film.
Preferably, an end of the resistive film along the direction of the slot is tapered. Impedance characteristics of the transmission line changes gradually, and signal reflection is thereby suppressed.
According to another aspect of the present invention, there is provided a nonreciprocal circuit apparatus including the above nonreciprocal circuit device, a yoke for forming a magnetic path by covering the periphery of the substrate, and a magnet for generating the DC magnetic field. With this arrangement, a nonreciprocal circuit apparatus is obtained which may be used as a miniaturized isolator having the substrate, the magnet, and the yoke integrated therein.
According to another aspect of the present invention, there is provided a transceiver including the above nonreciprocal circuit device or the nonreciprocal circuit apparatus.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.